The function of a closure cap to adequately seal the contents of a container against leakage from or into the container traditionally has been met by incorporating a soft liner to effect a seal between the under portion of the cap lid and the upper face of the bottle neck rim. The liner may be preformed from sheet or formed in place and is produced from materials or laminar combinations of materials which provide easy cold formability to enable the liner to conform to the individual configuration of the neck rim, including manufacturing aberrations and defects. Because of the specialized sealing function of a liner, it is typically made from softer polymers than those selected to perform the more structural cap functions of providing a strong resilient enclosure for the neck opening with a strong mechanical engagement therewith. In some instances stiffer and stronger polymers, including some which are suitable for producing threaded caps, may be foamed to produce an expanded, less dense sheet having a softer, more flexible characteristic and liners may be made therefrom.
An important characteristic sought for liners and not generally met, especially by plastic caps where the cap lid geometry and dimensions may be affected in time by internal pressure and/or heat exposure, is the ability to adjust to such dimensional changes without undue loss of sealing pressure. This calls for a liner with a high level of springiness and resistance to cold flow, particularly for carbonated and/or pasteurized foods and beverages employing plastic caps, to offset the large amounts of cold flow or creep which can result in a dome shape lid. Most soft, conformable liners by their nature will cold flow to adapt to the initial cap geometry but do not have the elasticity or resilience to adapt to such changing cap geometry and can lose their sealing engagement. An ideal liner, therefore, would possess a soft, easily conformable sealing surface, backed by a springy and resilient supporting structure contributing the good resistance to plastic creep to assure a good sealing engagement at all times under all conditions. Such an ideal liner could be vulcanized rubber which can possess both softness and resiliency over long time periods. However, the cost of such seals precludes their use in most applications. On the other hand, plastics which are suitably soft exhibit poor long term creep resistance and resilience. An alternative approach in popular use is a laminate of a springy paperboard substrate with a soft conformable sealing surface such as wax or plastic. This approach is good in theory but has performance limitations especially when moisture is present.
In any event, all cap liners add another component to the closure and significantly add to its cost.
Because of an economic advantage, attention has been devoted in recent years to developing caps which have an integral, "linerless" seal. The availability of such semi-rigid plastics as polypropylene and polyethylene, which combine a moderate level of strength and resilience with a moderate level of softness and conformability, has made possible popular use of caps with linerless seals. Typically, such caps employ a circular flange under the cap lid having a wedge shape cross section intended to abut the top surface of the bottle neck rim in a compressive action for sealing. The wedge shape flange generally is vertical and provides a sealing area restricted to the width of the narrower portion of the wedge shape. Such features provide a very limited sealing area resulting in reduced sealing integrity.
Other linerless caps employ circular flanges which present an angular cross section from the vertical so that capping will cause the flanges to flex and slide out over the top surface of the neck rim thereby creating a somewhat larger sealing area than obtainable with vertical flanges in straight compression. While the larger, though still limited sealing area has advantages, such cantilevered configurations concentrate the capping stresses in specific portions of the seal with resultant high localized creep and loss of sealing pressure. Another important limitation of such slanted linerless features is the difficulty of removing such features from an injection mold. This results in more complex and costly mold construction and operation and also excludes the more rigid plastics from use.
Still other linerless caps employ circular flanges with cross sections at an angle from the vertical which engage the corners of the neck rim with the underside of the flange. Such features rely on the use of very high sealing pressure directed against a restricted line contact at the rim corners to obtain sealing integrity. Again, however, capping stresses are concentrated and high with resultant high localized creep and loss of sealing pressure. Also, to the extent that the cross sections of such flanges approach the vertical, their sealing integrity is affected by out-of-round or other dimensional variations of the container manufacturing process or variations between manufacturers resulting from the fact that inside neck dimensions typically are not specified. And to the extent that the cross section of the flanges depart from the vertical, more complex and costly mold construction and operation result.
Still another type of linerless cap employs a plug configuration in sealing contact with the inside wall of the container neck. This type of seal has the advantage of engaging that surface of the bottle neck which is freest from manufacturing defects and most protected from incidental marring in handling thereafter. However, wide manufacturing dimensional tolerances and the industry-wide practice of not specifying the neck bore dimension impose severe limitations in trying to obtain consistent sealing engagement and integrity. As a result, tapered plug seals can push the cap lid up to varying degrees of undesirable dome shapes. Or plug seals can yield unacceptably wide variations in sealing engagement and pressures. Efforts to overcome such deficiencies have led to proposed designs with flanges extending radially from generally cylindrical plugs wherein the outer rim of the flange makes a narrow sealing contact with the neck bore and is supported by a cantilevered flexing action. (See, for example, U.S. Pat. Nos. 4,090,631, 4,016,996 and 4,210,251). Such designs also concentrate sealing stresses in restricted localized areas resulting in high localized creep and loss of sealing pressure in time. An additional problem has been encountered with this type of linerless seal in that the lip or rim of the flange may be distorted by the neck rim during capping leading to imperfect seals. Efforts to eliminate this problem can introduce other problems specific to pressurized containers wherein blow-off or missiling of the caps can occur during uncapping. Another effort to avoid distortion of the lip or rim of such a seal is a cap design and method of producing it wherein a radially extending flange having a downward orientation as molded is hingedly "bent", "folded" or otherwise oil-canned into an upward orientation before it is applied to the container. (See U.S. Pat. No. 4,210,251). This is accomplished with extra mold portions and actions during part removal or subsequently in an appropriate fixture to hingedly evert the flange. This effort, therefore, requires the molding of a seal of complex shape utilizing a complicated and costly mold construction and molding operations followed by everting the sealing portion of the seal hingedly to alter its orientation but not its shape.
Moreover, an inherent limitation to heretofore available linerless caps is that the sealing surface has the same plastic in the same physical state as the structural portion of the cap. This has called for a compromise in the softness and conformability of the sealing surface or in the strength of the structural cap portions, or most frequently both, with consequent limitations in the cap usefulness.
Thus, known caps with linerless seals are beset with drawbacks and problems associated with their need to perform with container necks having imperfect sealing surfaces and wide dimensional tolerances; their limited sealing integrity based on restricted sealing area and loss of sealing pressure over extended periods of time especially at elevated temperatures or with internal pressure or vacuum; the fact that sealing surface softness and conformability are limited; the fact that the use of more rigid plastics are not feasible; and the cost and complexity of mold construction and operation for a number of the proposed sealing designs.